Manual control of light trnsmission is generally required for many purposes, including industrial, commercial, and residential applications. One common application is the control of light passing through a window, skylight, or other opening for a variety of reasons, e.g., controlling visible light transmission, ensuring privacy, reducing heat build-up in a room, etc. Many types of curtains, venetian blinds, shades, draperies, shutters, and the like are used to control the amount of light admitted into a room or building through a window, skylight, or other opening.
In many cases, the devices used as light valves are totally or partially opaque and function by absorbing incident light. Because the light is absorbed by the devices, they typically convert light energy into heat, which can then be radiated into the building or space in which the devices are located. In addition, while some of the devices, e.g., venetian blinds and shutters, are effective at ensuring privacy, they may also excessively restrict the view through the window.
The use of polarizers as light valves in many different applications is well-known. Typically, two polarizers are used, at least one of which can be translated or rotated. In general, the transmission axis of the front polarizer is "crossed" with the transmission axis of the rear polarizer at angles that can vary between 0 and 90.degree. to adjust the amount of light transmitted through the polarizers. The rotation is typically about an axis that is normal to the plane in which the polarizer is located, i.e., the axis of rotation is out of the plane in which the rotating polarizer is located. As a result, the rotating light valves typically require that the window or other opening in which they are placed be generally circular. In other words, such devices are not particularly useful for windows which have a generally square or rectangular outline.
Other approaches at controlling light transmission through a window include the use of privacy films, such as those described in U.S. Pat. No. 5,686,979 (Weber et al.). Such privacy films use an electronically switched columnar dispersed liquid crystal film that provides reversible opacity with or without mechanical movement.
In addition, although the use of polarization to control the transmission of light has been discussed in many different manners, most of the discussions focus on the use of absorptive polarizers. Typically, absorptive polarizers use dichroic dyes which absorb light of one polarization orientation more strongly than that of the orthogonal polarization orientation. Because absorptive polarizers absorb light having one polarization orientation, they can also present the problems of conversion of absorbed solar energy into thermal energy which is then radiated into the interior of a room.
As a result, a need exists for a light valve that can a) control the amount of light passing through an opening, b) provide high contrast between its open and closed states, c) provide for absorption or reflection to improve energy efficiency, and/or d) provide control over viewing angle to either limit or expand viewing angles.